The invention relates to a process for generating a reference signal, which reproduces the vehicle speed, in a vehicle with automatically selectable two and four wheel drive and having an antilocking control system, as well as an arrangement for carrying out this process.
In the unpublished German Patent Application No. 3,505,455 (which corresponds to U.S. patent application Ser. No. 828,275 filed on Feb. 11, 1986 and assigned to the assignee of the present invention), an apparatus, and indirectly also a process, is described for a boost control system in a vehicle. This system operates to switch from a single axle to a two axle drive as soon as the drive slip of a permanently driven vehicle wheel exceeds a predetermined threshold value. In this manner, the traction of the vehicle is significantly improved, particularly when it is moving on a road with relatively low traction coefficients between the road and the driven vehicle wheels. This also applies to vehicles which are equipped with a permanent four wheel drive, as described, for example, in German Unexamined Patent Application No. 34 26 747, also not published, and in Automobiltechnische Zeitschrift, Journal for Automobile Engineering, Vol. 87, September 1985, pages 395-400. In such vehicles, which exhibit high traction even on roads with poor subsoils, it is very important to also optimize the brake system in the sense of good deceleration and driving stability so that the vehicle remains controllable in every operating situation, either acceleration or deceleration.
However, the combination of a four wheel drive with an antilocking system, as known, for example, through U.S. Pat. No. 3,950,036 or German Published Unexamined Patent Application No. 2,951,754 for vehicles with one axle drive, proves to be problematic at closer inspection. In such antilocking systems, the information about whether a decelerated vehicle has a tendency to lock, that means runs into an excessive brake slip, is obtained from the comparison of its actual circumferential speed, provided via wheel sensors, with a reference speed. This reference speed is taken as the measure for the vehicle speed and is supposed to represent the vehicle speed as closely as possible. In this process the following control algorithm is applicable: If the wheel speed is lower than the reference speed by more than a predetermined threshold value, the wheel is considered to be decelerated too much. The control electronics of the antilocking system then generates a pressure decrease signal to drive the brake pressure control valve by means of which the brake pressure is decreased in the wheel brake. In the conventional method, the brake pressure is decreased at this wheel until the wheel is accelerated again. This state is indicated by the fall of the characteristic (-b) signal for the deceleration of the wheel.
The brake pressure is then held constant at this wheel. For approximating the reference speed, the switch is once again made to pressure decrease, provided the wheel is not accelerated again. If, on the other hand, the wheel is accelerated, the switch is made to pressure increase as soon as the wheel speed is different from the reference speed by less than a threshold value. The object of this control is to hold the brake sliP of the vehicle wheels within the range of values that are compatible with good driving stability and good deceleration.
In the antilocking system of U.S. Pat. No. 3,950,036, at the start of deceleration, the speed of the least decelerated wheel is taken as the reference speed. The decrease in the reference speed is limited to the value -g (g=9.81 m/s.sup.2), as long as the reference speed is larger than the speed of all vehicle wheels. The reference speed is held constant if it is between the speeds of the fastest and the second fastest vehicle wheel. Moreover, the reference speed is the speed of the second fastest vehicle wheel, if at least one of the two fastest vehicle wheels is not driven, or the third fastest vehicle wheel, if the two fastest vehicle wheels are the driven vehicle wheels. Thus, the reference speed is limited "upward" by the speed of the slower of the non-driven vehicle wheels, whereas the decrease is limited by the value -g (cf. column 7, lines 20 to 48 of U.S. Pat. No. 3,950,036).
In the antilocking system shown in U.S. Pat. No. 3,950,036, only one single reference speed is produced with which the wheel circumferential speeds of all of the vehicle wheels are compared.
In contrast to this, in the antilocking system shown in German Published Unexamined Patent Application No. 2,951,754, a reference speed is produced for each vehicle wheel subjected to the antilocking system. This reference speed corresponds to a weighted average value of the wheel speeds of all of the vehicle wheels. In generating the average value, the largest weight is given to the vehicle wheel for which the aforementioned reference speed is generated. In addition to this, the non-driven vehicle wheels, having a higher weighting, enter into the formulation of the average value as the driven wheels (cf. page 11, par. 1 of the German Published Unexamined Patent Application No. 2,951,754). Moreover, in that German reference, which agrees with U.S. Pat. No. 3,950,036, it is proposed that the reduction in reference speeds be limited to a maximum fixed value. In generating the reference speed assigned to the individual vehicle wheels, adequate consideration should be given to the fact that on a road having a number of curves, the individual vehicle wheels generally roll along different radiis of the curve and thus their circumferential speeds should also be compared with different vehicle speeds. Further, the process disclosed by the German Published Unexamined Patent Application No. 2,951,754 for generating different reference speeds for individual vehicle wheels, is different from the process disclosed by U.S. Pat. No. 3,950,036 for generating a reference speed, which is regarded as representative for the entire vehicle. This is due to the fact that the maximum value of the reference speed(s) in the extreme case corresponds to the greatest wheel speed possible with which the vehicle wheels can spin.
However, both known antilocking systems have in common that at the start of the control phase, the standard reference speed agrees or nearly agrees with the amount of a wheel speed obtained at that time.
This is not a problem provided the reference speeds that are the determining factors at the start of the control process can be obtained from wheel speeds of non-driven vehicle wheels, as is the case in the two aforementioned explained antilocking systems, since these wheel speeds usually give a good approximation of the vehicle speed.
However, this does not apply to vehicles with a permanent four wheel drive or one that is only periodically switched on, since in such vehicles it is possible to have operating situations in which all of the vehicle wheels spin uniformly. In that case, there no longer exists a definite relationship between the vehicle speed and the wheel circumferential speed. A typical situation of this nature is, for example, uphill driving on a road having a uniformly low traction coefficient between the road and the vehicle wheels. In such a situation it is possible for the vehicle to move with significantly less speed than the circumferential speed of the driven vehicle wheels. And in the extreme case it might even occur that the vehicle slides backwards even though all of the vehicle wheels are driven in the forward direction and thus spin at a high speed.
Such a situation can only be controlled by activating the brake if the brake system responds directly after its activation, as is the case, for example, in a brake system without an antilocking system. If the vehicle, however, is equipped with an antilocking system of the type described above, a non-actual high speed value by which a corresponding high vehicle speed is approximated, is applied as the reference speed for the antilocking control system if the brake is activated. The antilocking system responds immediately after the start of deceleration, since the vehicle wheels, spinning up to that time on a smooth subsoil, are decelerated very quickly, so that the wheel decelerations can be significantly larger than the rate of change of lg anticipated for the reduction of the reference speed. This rate of change corresponds to the maximum value of the vehicle deceleration which may be obtained under good road conditions.
The antilocking control system responds to the high values of the wheel decelerations and/or to the difference between the wheel speeds and the respective reference speed, this difference having set in after a short period of time and being already too large. Since the wheel speeds fall far below the reference speed, a very high brake slip value is generated by the antilocking system with the result that the control system reacts by decreasing the pressure in the wheel brakes. The brake system is practically out of action. This state continues until the reference speed at this limited rate of change has dropped so far that the antilocking control system again "sees" equivalence of the reference speed and the wheel speed. This combination corresponds with an increase in the brake pressure and triggers the build-up in its pressure increase position by switching back the brake pressure control valve.
In order to prevent such a "failure mode" of the antilocking system in a vehicle with permanent all wheel drive, in the ATZ Sept. 9, 1985, pages 395 to 400, (in particular page 400, line 5,) it is proposed that the longitudinal deceleration in the vehicle be detected in order to be able to obtain information regarding the friction level between the road and the vehicle wheels and the criteria for adequately controlling the brake pressure. However, the longitudinal acceleration sensor required for this detection has, by definition, the disadvantage that it also reacts to the rise and fall of the road. This, in turn, undesirably results in an antilocking control system having control characteristics which vary widely and are dependent on the road condition.
In a vehicle, having automatic selectable all wheel drive, as described in German Published Unexamined Patent Application No. 3,505,455, or in a vehicle, generally having a permanent all wheel drive or an all wheel drive that can be at least partially switched off, as described in the German Published Unexamined Patent Application No. 3,426,747, theoretically the above-described "failure mode" of an antilocking control system could be prevented by carrying out test cycles. These test cycles would involve periodically switching the all wheel drive off, for short time spans. In the course of these tests the non-driven vehicle wheels can assume the actual vehicle speed, which is used as a supporting value for adjusting the reference speed. In principle it would be possible to accurately adjust the reference speed to the actual vehicle speed. Such a measure, however, would result in a most uncomfortable, vibration-like acceleration and deceleration behavior of the vehicle due to the constant switchover from one axle to two axle drive and vice versa, which is required for the adjustment of the reference speed. Such behavior would reduce the utility of the all wheel drive and would not be acceptable.
Thus, an object of the present invention is to provide in a vehicle, which is equipped with an automatically selectable all wheel drive and an antilocking system which operates according to a conventional control system logic, dependent on the threshold values of the wheel decelerations of the brake slip, an arrangement which provides not only a fast response of the brake system in decelerating the vehicle wheels but also guarantees the maintenance of the antilocking control function. The invention also provides a process carried out by the arrangement for providing the functions listed above.
The objects of the present invention are attained by providing a process in which a reference signal is generated in which the reference signal is increased at a rate which is limited by a predetermined threshold rate if both axles are drive coupled via the longitudinal differential and the wheel speed signals increase faster than the threshold rate. In certain preferred embodiments, the process also includes the steps of: keeping the reference signal at a maximum value attained by the reference signal during a process of locking the longitudinal differential; periodically uncoupling an axle from a power train of the vehicle for a test cycle, and increasing the reference signal at a predetermined test cycle rate if the largest of the wheel speed signals is greater than the reference signal; and reducing the reference signal down to the largest wheel speed signal if the reference signal is greater than the largest speed signal.
Accordingly, in an operating state in which the all wheel drive is already switched on, but the longitudinal differential of the power train is not yet locked, as well as in the course of the test cycles, which are conducted to detect possible differences between the reference speed and the vehicle speed, the reference speed is adjusted to the highest speed of the wheel circumferential speeds at an increased rate of change. For the duration of the operating states of the vehicle in which the longitudinal differential of the power train is locked, the reference speed is restricted. The value of the reference speed at the time the longitudinal differential lock was switched on serves as the upper limit. In driving situations in which the vehicle is driving on a "poor" subsoil so that there is a low traction coefficient between the road and the driven vehicle wheels, this upper limit prevents the reference speed from rising to unrealistically high values.
By quickly adjusting the reference speed signal to the highest of the wheel speeds, it is possible to maintain the antilocking control function in practically all relevant cases, if the reference speed is larger than the highest speed of the wheel speeds when the antilocking control system sets in. The antilocking function, in turn, should be able to decelerate the vehicle. On the whole, for the statistically significant driving and decelerating situations, the present invention makes it possible for the braking system to respond adequately fast after its activation and to allow deceleration with the use of the antilocking control functions.
In preferred embodiments of the process, by matching the reference signal to the wheel speed signal after a delay time within which the vehicle wheels attain operating speed, the reduction in the reference signals, resulting from an antilocking controlled deceleration, results in the reference speed signal being quite accurately adjusted to the vehicle speed.
In a further preferred embodiment of the process, the duration of a delay time span of between 80 to 200 milliseconds has been ascertained by systemic tests as particularly advantageous. When the time span has ended, the non-driven vehicle wheels assume with high probability the vehicle speed.
In a still further preferred embodiment of the process, a favorable rate of change for adjusting the reference speed to the wheel speed when the reference signal is matched to the largest speed signal following the time delay is at least -80 m/s.sup.2. This rate of change can be realized by way of simple means in the digital electronic circuit technology.
By providing a reference speed signal that is changed incrementally, in certain preferred embodiments a realistic reproduction of the vehicle speed through the reference speed, in particular, when driving uphill with low traction coefficients between the road and the driven vehicle wheels, is attained.
The arrangement for carrying out this process has an all wheel drive control for placing the vehicle in an all wheel drive mode and locking the longitudinal differential, and an antilocking control system. A reference signal generator produces a reference signal and is controlled by three operational state controllers to cause the reference signal generator to either increase, decrease or hold the reference signal produced depending on detected conditions.
An advantageous feature of the arrangement is the simple electronic switching arrangement for the logical switching of output signals of the electronic control units of the antilocking system and the drive control for the comparison of the reference speed, generated in the manner of the invention, with the wheel speeds of the vehicle, as well as for a time delayed adjustment of the reference speed to the wheel speed. The specific arrangement could also be designed as auxiliary switching units for conventional control units for all the wheel drive control system and the antilocking control system.
These and other objects, features, and advantages of the present invention will become more apparent from the following description when taken with the accompanying drawings which show, for purposes of illustration only, one embodiment in accordance with the present invention.